The present application claims foreign priority from Japanese Patent Application No. Hei 11-013515, filed on Jan. 21, 1999, the content of which is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a spark plug used as a source of ignition in an internal combustion engine, and more particularly, to a spark plug having a small-sized metallic shell for installation in a narrow space.
2. Description of the Related Art
Some conventional spark plugs employ a cushion material formed from talc powder and filled into a cylindrical space defined by an outer circumferential surface of an insulator and an inner circumferential surface of a metallic shell to improve impact resistance. Other conventional spark plugs do not employ such a cushion material (talc) but are configured such that the insulator is secured directly by the metallic shell through thermal swaging. These conventional spark plugs include a screw diameter of 14 mm (M14) or 12 mm (M12), and a hexagonal tightening portion with which a plug wrench is engaged having a distance of 20.8 mm or 16 mm between two parallel, diagonally opposed surfaces of the hexagonal tightening portion (xe2x80x9cwidth-across-flatsxe2x80x9d).
With recent improvement in engine control technology a tendency toward employing a multi-valve type combustion chamber, the number of components mounted on and around an engine has been increasing. Particularly, in the case of a direct-injection-type engine, which is becoming popular, a volume allot ed to a spark plug on a cylinder head is small. Accordingly, the width-across-flats of a tightening portion of a metallic shell has been required to be decreased from the conventionally employed width of 16 mm to not greater than 14 mm.
When the width-across-flats is reduced to not greater than 14 mm, the wall thickness of a metallic shell decreases accordingly. As a result, the volume of the metallic shell decreases, and thus, the metallic shell decreases in strength. A spark plug having a width-across-flats not greater than 14 mm and not employing a cushion material (talc) suffers impairment in impact resistance and a considerable reduction in airtightness after exposure to impact.
Also, since the wall thickness of a tightening portion decreases, a load imposed on the tightening portion during swaging causes swelling of the tightening portion. As a result, the width-across-flats may fail to fall within a predetermined tolerance, potentially causing a failure to establish engagement between the tightening portion and a plug wrench.
The aforementioned engagement problem will be described specifically with reference to FIGS. 2 and 3. An insulator 1 is fixedly attached to a metallic shell 5 by swaging in the following manner. A swaging die is applied from underneath to a seat portion 5F of the metallic shell 5, while another swaging die is applied from above to a tightening portion 5A and a swaging portion 5C. The upper swaging die exerts a downward force to buckle a curved portion 5D by about 0.5 mm to 0.8 mm, whereby the insulator 1 is strongly pressed against an inner stepped portion 5E of the metallic shell 5 via a packing member 6. In this manner, the insulator 1 is fixedly attached to the metallic shell 5 through swaging. During such swaging, a strong force exerted by the upper swaging die causes plastic deformation of not only the curved portion 5D but also the tightening portion 5A. As a result, the tightening portion 5A swells slightly. This swelling has not raised a problem with respect to a conventional spark plug having a width-across-flats W of not less than 16 mm, since a wall thickness P of the tightening portion 5A is sufficiently thick so that the tightening portion 5A has a sufficient strength.
However, a spark plug having a width-across-flats of not greater than 14 mm encounters a difficulty in bringing the width-across-flats W within a predetermined tolerance, since the wall thickness P of the tightening portion 5A is thin and results in significant swelling of the tightening portion 5A. Unless the width-across-flats W falls within a predetermined tolerance, a plug wrench cannot be engaged with the tightening portion 5A. By contrast, when, in order to reduce the swelling of the tightening portion 5A, the wall thickness of the curved portion 5D is reduced so that a forced required to buckle the curved portion 5D can be reduced, the strength of the curved portion 5D of a spark plug becomes insufficient for enduring a tightening torque exerted when the spark plug is mounted on an engine. Alternatively, when a thickness M of talc 9 serving as a cushion material is reduced to accordingly increase the wall thickness P of the tightening portion 5A, the effect of the talc 9 as a cushion material is diminished, resulting in an impairment in impact resistance.
An object of the present invention is to provide a spark plug capable of exhibiting high impact resistance even when the width-across-flats of a tightening portion of a metallic shell is small, and capable of maintaining airtightness even after subjection to strong impact.
Another object of the present invention is to provide a spark plug having further improved impact resistance and capable of bringing the width-across-flats of a tightening portion into a predetermined tolerance through suppression of swelling of the tightening portion.
A further object of the present invention is to provide a method of manufacturing a spark plug as mentioned above.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages and purpose of the invention will be realized and attained by the elements and combinations particularly pointed out in the appended claims.
To achieve the above objects, the present invention provides a spark plug including an insulator having a center through-hole formed therein, a center electrode held in the center through-hole, a metallic shell holding the insulator by swaging, and a ground electrode electrically connected to the metallic shell and defining a spark discharge gap in cooperation with the center electrode. The metallic shell has a male-threaded portion formed on the outer circumferential surface of a front end portion of the metallic shell, and a tightening portion formed on the outer circumferential surface of the metallic shell, located at a rear side with respect to the male-threaded portion. In the specification, the term xe2x80x9cfrontxe2x80x9d refers to a spark discharge gap side with respect to an axial direction of the center electrode, and the term xe2x80x9crearxe2x80x9d refers to a side opposite the front side.
The tightening portion is used to screw the male-threaded portion into a female-threaded hole formed in an internal combustion engine. The distance between two opposed parallel surfaces of the tightening portion (hereinafter referred to as a width-across-flats W) is not greater than 14 mm (Wxe2x89xa614.0 mm).
A cushion material is charged into a cylindrical space defined by an outer surface of the insulator and an inner surface of the metallic shell to thereby form a cushion-material charged portion. The cushion-material charged portion has an axial length L of from 0.5 mm to 10.0 mm inclusive (0.5 mmxe2x89xa6Lxe2x89xa610.0 mm) and a thickness M of from 0.5 mm to 1.3 mm inclusive (0.5 mmxe2x89xa6Mxe2x89xa61.3 mm). The cylindrically filled cushion material eases impact exerted on the metallic shell, thereby preventing loosening of swaging between the metallic shell and the insulator even when the width-across-flats is not greater than 14 mm. Even when swaging between the metallic shell and the insulator loosens to some extent and thus the pressure produced at the packing potion between the metallic shell and the insulator decreases with a resultant leakage of combustion gas through the packing portion, the cushion-material charged portion serves as a second packing to prevent leakage of the combustion gas from the spark plug.
When the axial length L of the cushion-material charged portion is less than 0.5 mm, the cushion-material charged portion fails to effect cushioning as expected. When the axial length L of the cushion-material charged portion is in excess of 10 mm, the cushion material cannot be sufficiently filled into the cylindrical space. The resultant cushion-material charged portion has a low cushion material density and thus fails to effect cushioning as expected. When the thickness M of the cushion-material charged portion is less than 0.5 mm, the cushion-material charged portion fails to effect cushioning as expected. When the thickness M of the cushion-material charged portion is in excess of 1.3 mm, the wall thickness of the tightening portion of the metallic shell decreases accordingly, resulting in an impairment in the strength of the metallic shell.
Accordingly, even when the width-across-flats is not greater than 14 mm, the spark plug endures use at high temperature and exhibits excellent impact resistance.
Preferably, the metallic shell has a seat portion located between the male-threaded portion and the tightening portion and has a diameter greater than that of the male-threaded portion, and a curved portion which extends between the tightening portion and the seat portion. The curved portion is buckled through axial swaging while being heated to integrate the metallic shell and the insulator into a single unit.
Through employment of the above-mentioned swaging combined with heating; i.e., hot swaging, a load required for swaging; i.e., a load required for buckling of the curved portion, becomes smaller than that required for cold swaging. Therefore, the load imposed on the tightening portion during swaging is decreased accordingly. Even in the case of the tightening portion having a thin wall thickness, swelling of the tightening portion becomes sufficiently small so as to bring the width-across-flats within a predetermined tolerance. Also, when the heated curved portion cools after swaging, the curved portion shrinks axially, so that the pressure produced at the packing portion through swaging further increases to thereby improve airtightness of the spark plug.
Notably, whether a spark plug has been formed through employment of hot swaging or cold swaging can be easily determined through analysis of a halved piece of the spark plug. In a spark plug formed through employment of hot swaging, a buckled curved portion exhibits swelling in radially inward and outward directions; i.e., the curved portion is deformed such that the thickness thereof is increased. By contrast, in a spark plug formed through employment of cold swaging, the buckled curved portion is deformed in either a radially inward direction or a radially outward direction.
The present invention further provides a method of manufacturing a spark plug including an insulator having a center through-hole formed therein, a center electrode held in the center through-hole, a metallic shell holding the insulator through swaging, a ground electrode electrically connected to the metallic shell and defining a spark discharge gap in cooperation with the center electrode, and a ground electrode electrically connected to the metallic shell and defining a spark discharge gap in cooperation with the center electrode. The metallic shell has a male-threaded portion formed on the outer circumferential surface of a front end portion of the metallic shell and a tightening portion formed on the outer circumferential surface of the metallic shell, located at a rear side with respect to the male-threaded portion. The tightening portion is used to screw the male-threaded portion into a female-threaded hole formed in an internal combustion engine.
The method includes the steps of: forming the metallic shell such that the distance between two opposed parallel surfaces of the tightening portion (hereinafter referred to as a width-across-flats W) is not greater than 14 mm (Wxe2x89xa614.0 mm) and that the metallic shell has a seat portion located between the male-threaded portion and the tightening portion and has a diameter greater than that of the male-threaded portion, and a curved portion that extends between the tightening portion and the seat portion; charging a cushion material into a cylindrical space defined by an outer surface of the insulator and an inner surface of the metallic shell to thereby form a cushion-material charged portion having an axial length L of from 0.5 mm to 10.0 mm inclusive (0.5 mmxe2x89xa6Lxe2x89xa610.0 mm) and a thickness M of from 0.5 mm to 1.3 mm inclusive (0.5 mmxe2x89xa6Mxe2x89xa61.3 mm); and pressing the tightening portion and the seat portion toward each other while applying current thereto so as to heat the curved portion, to thereby buckle the curved portion.
Through employment of the above-mentioned steps, a load required for swaging can be decreased. Thus, a spark plug manufactured by the above method yields the effects mentioned previously. Also, swelling of the tightening portion can be reduced to a practically acceptable extent.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.